This invention relates to the provision of insulative protective coatings on substrates, and more particularly to a novel process for providing a tough, thin, adherent, insulative coating that constitutes a barrier against penetration of liquid water and ions to the surface of the substrate.
Numerous applications require protection of a substrate against contact with liquid water, ions or ionizable species. In many mechanical or electrical devices, contact of the surface with moisture may cause galvanic corrosion. In electrical devices such as integrated circuits, protection of the substrate surface from moisture may be essential to prevent the generation of stray current paths which may result in short-circuiting of the device.
Insulative or other protective materials which work well in dry air are often inadequate in high humidity or liquid environments. Both corrosion and electrical breakdown problems are exacerbated if a device is exposed to water that contains inorganic ions. One particularly difficult and sensitive application of integrated circuits, semiconductors and metal electrodes is in electrical or electronic device implantation in a human or animal body. Extra-cellular fluids within the body are saline, and often contain a number of other ions or other electrolytes. At body temperatures, severe and rapid corrosion may lead to rapid and untimely failure of the device. Short of corrosive failure, the operation of the device may be disturbed by stray currents in a manner which can, in some instances, be catastrophic to the host.
Various materials have been developed to provide an electrically insulative moisture barrier over a substrate. Among the more prominent of these are aromatic polyimides such as those sold under the trade designation "Kevlar" by E.I. DuPont de Nemours, & Co. However, polyimides must be applied by wet processes such as deposition from organic solution, requiring the handling, recycle, and/or disposal of organic solvents, and thus implicating the materials cost, environmental control, and capital and operating costs that attend solvent deposition processing. Moreover, polyimide coatings must be cured by baking, and the polyimide coating process generally requires close control of process parameters.
Another group of materials which have good insulative properties and reasonably good resistance to moisture penetration are the vapor deposition polymers referred to as parylenes. Parylene N coatings are produced by vaporizing a di(p-xylylene) dimer, pyrolyzing the vapor to produce p-xylylene free radicals, and condensing a polymer from the vapor onto a substrate that is maintained at a relatively low temperature, typically ambient or below ambient. Parylene N is derived from di(p-xylylene), while parylene C is derived from di(monochloro-p-xylylene), and parylene D is derived from di(dichloro-p-xylylene).
Although parylenes have generally advantageous electrical, chemical resistance and moisture barrier properties, it has been found that these polymers do not adhere well to many substrate surfaces, particularly under wet conditions. Although these polymers are quite resistant to liquid water under most conditions, they are subject to penetration by water vapor which may condense at the interface between the parylene film and the substrate, forming liquid water which tends to delaminate the film from the substrate. Vapor deposited parylene films are also generally quite crystalline and are subject to cracking which may also create paths for penetration of moisture to the substrate surface.
Substantial efforts have been made in the art to devise means for pre-treating a substrate to enhance adherence to subsequently applied parylene. Other work has related to the treatment of a pre-formed parylene film to improve its adhesiveness to another surface to which it is subsequently mated.
One factor which has reportedly affected the adherence of parylene is the hydrophobicity of the substrate. Certain of the work previously conducted in this art has involved plasma treatment of the substrate, which has been found to render the substrate more susceptible to adhesion of subsequently applied vapor deposited parylene. As found, for example by Sharma et al., "Effect of Surface Energetics of Substrates on Adhesion Characteristics of Poly-p-xylylenes", J. Adhes., 1982, 13(3-4), 201-14, glow discharge treatment of a glass surface with an argon or methane plasma reduces the surface energy and leaves the substrate surface in a hydrophobic condition. Sharma et al. suggest the application of a glow discharge polymerized methane primer coating to the substrate prior to vapor deposition of the poly-p-xylylene. They also investigated preliminary glow discharge treatment of the surface with either argon or oxygen, but found treatment with methane plasma to be the most effective. Oxygen plasma treatment was found to render the surface hydrophilic and was detrimental to adhesion. In the course of Sharma et al.'s experimental work, the walls of the reactor became coated with parylene and some of this material was sputtered off and deposited on the substate during plasma treatment. However, this effect was reported to be much less pronounced during treatment with methane than with argon.
Sharma et al, "Effect of Glow Discharge Treatment of Substrates on Parylene-Substrate Adhesion", J. Vac. Sci. Technol., 21(4), Nov./Dec. 1982 also describes the pretreatment of metal and glass surfaces with argon, oxygen or methane plasma prior to the vapor deposition of poly-p-xylylene. Consistent with the findings reported in the J. Adhes. article discussed above, the authors in the J. Vac. Sci. Technol. article found that treatment with methane plasma was the most effective for wet and dry adhesion of parylene, and that oxygen treatment was undesirable for such purpose. It was reported that treatment with methane plasma resulted in the formation of a thin hydrophobic glow discharge polymerized layer on the substrate, and that this layer contained free radical sites for covalent bonding with the parylene. It was further pointed out that provision of a glow discharge polymerized methane primer coating could be effected in the same vessel in which the vapor deposition of parylene takes place. Wet and dry adhesion tests were reported for various nonporous substrates, including poly(tetrafluoroethylene), polypropylene, polyethylene, poly(methyl methacrylate), poly(ethylene terephthalate), nylon-6, and glass, and certain porous substrates, including "Gorotex" and "Millipore" (0.25 micron). Porous substrates showed enhanced adhesion due to penetration of the polymer coating into the pores, and mechanical interlocking of the polymer film with the pores of the substrate. Plasma treated platinum foil was subjected to Auger analysis to provide information relating to the composition of the treated surface.
Nichols et al, "Evaluating the Adhesion Characteristics of Glow-Discharge Plasma-Polymerized Films by a Novel Voltage Cycling Technique", J. Appl. Polymer Sci.: Applied Polymer Symposium, 38, 21-33 (1984) describes the use of modified cyclic voltammetry techniques to test the adhesion of parylene films to substrates. In applying the parylene films to the substrates, the authors subjected the substrate to plasma pretreatment. Cyclic voltammetry tests verified that the use of primer coatings of glow-discharge polymers improved the adhesion of parylene films to platinum substrates. This article further states that previously used tests, such as pull rod tests, had been carried out on relatively large substrates, and are substrate and geometry dependent. The cyclic voltammetric test was developed to provide more reliable test data for coatings on micron-size wire probes such as might be used for stimulating and/or recording neural electrodes.
While the deposition of a primer coating by glow discharge polymerization of methane has thus been shown to provide a material improvement in the adhesion of parylenes to various substrates, it has further been found that such a primer is not, by itself sufficient to provide completely reliable long-term adhesion for parylene coatings on such products as implantable electrodes or integrated circuits. The glow discharge poly(methane) layer is very densely crosslinked, which tends to make it exceptionally resistant to moisture condensation. However, the glow discharge polymerized poly(methane) (GDM) layer is also necessarily very thin, which allows paths for tunneling currents to penetrate to the substrate surface. Moreover, because of its high cross-linking density, the GDM layer is highly stressed internally. Consequently, if the thickness of the glow discharge GDM layer is increased beyond about 50 angstroms, it tends to crack or craze, leaving paths both for passage of current and penetration of ions and liquid water. Furthermore, despite the report of the Sharma et al J. Vac. Sci. Technol. that GDM provides free radical sites for the bonding of a subsequently applied parylene coating, the free radicals at the free surface of a methane glow discharge polymer film are relatively few in number and tend to be extinguished by reaction with each other and atmosphere oxygen. Accordingly, the density of free surface and bulk free radicals in a GDM primer coating is not high to begin with, and decays rapidly with time or exposure to air. As a result, the bonding strength between the GDM and vapor deposited parylene layers is not exceptionally high, and the mean distance between interfacial free radical bonds may be larger than the dimensions of integrated circuit components, so that ion-containing condensed phase water may coalesce to droplet sizes large enough to cause stray currents on, along, or across components on the substrate surface. Thus, encapsulation of an implantable electrode or integrated circuits by application of a GDM primer, followed by vapor deposition of parylene, has not been demonstrated to reliably provide for long term operation in the presence of extra-cellular fluids.
In the work described in the above referenced articles, the glow discharge polymerization was carried out in an audio to radio frequency field established by capacitance coupling using aluminum electrode capacitors. In such a system, the electrodes of the capacitor are conventionally located inside the glow discharge polymerization chamber. As a result, aluminum metal is sputtered off the surface of the electrodes and deposited with the GDM on the surface of the substrate to be coated. This phenomenon has so characterized the capacitance coupled glow discharge research work that it led to the question of whether the presence of the aluminum in the coating film might be a critical contributor to the high degree of adhesion of the GDM to the underlying substrate, particularly in the case where that substrate is an inorganic material. H. Yasuda, Plasma Polymerization, Academic Press (1985), pp. 193-194. However, whatever its effect may have been on adhesion, the presence in the coating of aluminum, a highly conductive material, is not desirable where the substrate comprises an electrode, integrated circuit, or other electronic component.